We have news tonight about al qaeda and its targets and a possible contact here in the united states. Abc's chief foreign affairs correspondent reporting in. Reporter: We have learned about al qaeda's main leader after osama bin laden's death and the head of al qaeda in yemen. Abc news has also learned tonight why this threat that was emanating from overseas has now caused such alarm in the u.S. Tonight, beefed up law enforcement, heightened awareness at airports from l.A. To new york, and more scrutiny on those trying to enter the country. A senior u.S. Official tells abc news that the plot started in yemen and tonight u.S. Officials are franticly searching for vehicle bombs al qaeda wants to use to blow up the u.S. Embassy there and perhaps others as well. Abc news has also learned that through surveillance and electronic eavesdropping more was discovered, including communications between al qaeda fill yants and someone in the u.S., Not just electronic conversations but through the mail as well. The u.S. Does not know the content of the letters. In terrorist communications in general, the simpler more primitive forms of communication, the more effective they are. Reporter: This is who al qaeda has been communicating with in yellen. One of those behind the under wear bomb. Small bombs that could be placed inside a human body, especially dangerous if the bomb carrier boarded a flight. This group is fairly ingenius, fairly bold and eager to cause damage. Reporter: Despite the long war an al qaeda in places such as afghanistan and pakistan, there are affiliates are now spread out across the map in yemen, syria, throughout africa as well as iraq and libya where in recent days more than 1,000 of the world's most dangerous prisoners were busted out of jail. This is a wakeup all. Sk is stronger than it was in 9/11 because it's mute tatd and spread. Reporter: Officials strongly dispute that today saying it's much harder for these splinter groups to strike out globally. The white house spokesman reiterated this is a very serious threat. How will they know to stand down, that the alert is over? They think this is the critical week and that whatever the exact plans were, al qaeda would have to complete the operation or risk getting caught. So this week the key week, thank you martha raddatz. Terrorism and the whole question what is terrorism will arise tonight in texas as two sides gear up for one of the biggest military trials in decades. Major nidal hassan charged with murder for his rampage at fort hood back in 2009. He will be defending himself. Abc's chief investigative correspondent brian ross reports that many of those he injured will be there to con front him and also to confront the army for what happened since. Reporter: For the victims who will be called to testify, the trial means reliving the horror of what happened that day at fort hood, seen in this footage, obtained by abc news, taken moments after the shooting stopped. Everyone is scrambling. It's chaos. A few seconds after he started shooting is when I took a round to the chest. Reporter: The victims will also face cross-examination by the accused shooter himself, major nidal hassan, who has opted to serve as his own lawyer in the military trial. But even more upsetting, and what has led many of the victims to file this lawsuit, is the decision by the army to deny the injured and dead soldiers a purple heart, treating the fort hood shooting simply, "workplace violence." Recently retired staff sergeant shawn manning, who will testify and still has bullets lodged in his body, says that means lower priority veterans medical care and a loss of tens of thousands of dollars in benefits. Basically, they're treating us like I was downtown and I got hit by a car. Reporter: The accused shooter, nadal hassan, identified in this photo by three of the victims allegedly shouted a la act bar before he shot. He had 7 rounds on his person. Reporter: The secretary of the army says no victims ever received substandard care. To declare foreign terrorists would have a potentially profound effect on the ability to conduct the trial. Brian ross, abc news.

This transcript has been automatically generated and may not be 100% accurate.